Cutting trajectories at a particular time point
I have some mouse tracking trajectories where participants hear "Click on the X" where X is a word, that changes across a number of trials. As this is naturalistic speech, the length of the carrier phrase 'Click on the' is variable, and I want to remove the portion of the trajectories that before participants hear X.
I have the lengths of the carrier phrases stored in a data file. I can match across mouse tracking data (mt_data$data) and trajectories (mt_data$trajectories) by mt_id, so no problem there. However, I don't know how to cut the trajectories, being unfamiliar with arrays.
How would one go about this? Having performed this analysis, only then would I want to calculate measures etc